UnitedHealth posted a better-than-expected fourth-quarter profit, but a nagging rise in medical costs and care utilization surprised Wall Street.

UnitedHealth's high medical costs and lower-than-expected revenue in the fourth quarter underscored persistent challenges at its insurance unit, sending the healthcare conglomerate's shares down about 4% before the bell on Thursday.

UnitedHealth Group recorded slightly higher earnings in the fourth quarter, though its sales gains fell short of analyst projections.

UnitedHealth Group Inc. reported fourth-quarter profit that surpassed Wall Street’s expectations in its first financial statement since a top executive was killed in New York last month.